The present invention relates to a method for the recovery of gold value from various source materials containing gold in a low content by separating from other foreign metallic ingredients to isolate gold in a relatively high purity.
Needless to say, gold is the most valuable precious metal and not only is used traditionally as a material of a wide variety of ornamental goods but also is used recently as a key material in various kinds of electronic devices and instruments. It is unavoidable in the manufacturing process of these ornamental goods and electronic items using gold as a part that scraps of gold-made parts are produced more or less as a waste material mainly consisting of other less valuable metallic materials. In addition, as a consequence of the remarkably rapid progress of the technology of electronics, many of the electronic instruments are necessarily replaced with a new model after a relatively short time of use so that the amount of gold value contained in the obsolete and discarded instruments is also rapidly increasing year by year. In view of the outstanding expensiveness of gold as a precious metal, it is a very important industrial problem to develop an efficient method for the recovery of gold value from those waste materials of low gold content in a purity sufficiently high for the reuse of the gold metal.
One of the conventional methods for the recovery of gold value from waste materials is to dissolve the gold value in aqua regia or in an aqueous solution of a cyanide compound saturated with air blown thereinto followed by reduction of the gold ions into a metallic form. These methods, however, are not quite satisfactory as an industrial method because aqua regia and cyanide compounds are notoriously harmful or toxic against human body involving a serious problem on the workers' health or heavy environmental pollution.
A proposal is recently made in Japanese Patent Kokai No. 4-6229 and No. 4-21726 on a method for the recovery of gold value without using such a harmful material, in which the gold value in waste materials is dissolved in an organic solvent containing halogen in an elementary form and a halogen compound followed by re-precipitation of gold therein by the reduction with a reducing agent such as zinc dust, sodium borohydride and the like.
This method is also disadvantageous because the gold value dissolved in the medium can be precipitated only by undertaking the reducing reaction so that the process is very complicated. Moreover, the use of a reducing agent means production of a large amount of by-products therefrom as a contaminant of the recovered gold value which must be subjected to a further purification treatment in order to be in a high purity of gold suitable for reuse.